fence and climbed over it. The charged wire on 
the top of this barrier fence was not in operation 
at the moment, but even so this was a highly 
unusual behavior since after the first few experiences 
with the electric fence, rats normally avoid climb- 
ing fences. After wandering around in Area I it 
again climbed over the barrier fence and went to 
its home burrow, the South Alley Burrow. This 
return home may have been purely a chance one. 
This orientation toward its home despite the re- 
duction of utilization of tactile and visual stimuli 
is interpreted as indicating some means of orienta- 
tion in which learned patterns of behavior are not 
necessarily requisite. 
Example No. 3 (fig. 73B): This example involves 
the interaction of the sense of smell and conditioned 
routes of travel on the location of a new source of 
food. Details are presented in the following 
section. 
f. The interaction of the sense of smell and conditioned 
routes of travel upon the ability to locate a new source of 
food. Between January 24 and 30, 1948, there 
was a continuous blanket of snow over the ground. 
As was usual when there was a cover of snow, the 
routes of travel and the places of residence became 
more restricted. During this period the place of 
living of the rats was confined to the west half of 
Area I, the South Alley Burrow, the North Alley 
Burrow, a burrow at Passage 3, the east half of 
Area III, and the north half of Area II. For all 
practical purposes there was a single trail developed 
from Area I, through Passage 1, by the South 
Alley Burrow, through or around the Food Pen, 
by the North Alley Burrow, to and through Passage 
3 and thence over into both Areas II and III. 
With the exception of two round trip passages 
between Box 32 and Passage 1 there had been no 
movement into Area IV. This presented an 
excellent opportunity for an experiment on orien- 
tation by rats. To this end a small pile of garbage 
from my home was placed in the extreme west 
corner of the pen just west of Box 36. This was 
placed at 5 p.m. on January 29, 1948. The rats 
were not observed that night but the trails (fig. 
73B) the next morning revealed the nature of 
their behavior. On the morning of January 30 
there were well worn trails from Passage 1 to the 
garbage and from both the North Alley Burrow 
and the northwest outside corner of the Food Pen 
to Passage 4 and thence around through Area 
IV to the garbage. This might also apply to the 
South Alley Burrow rats since they frequently 
made excursions over into Area I through Pas- 
sage 1. However, one new trail developed during 
the night is most revealing in regard to the inter- 
action of the responses which led the rats in the 
rest of the pen to the new source of food. From 
the outside west side of the Food Pen, at a point 
half way between Passage 8 and the northwest 
corner of the Food Pen, a trail traveled by at 
least 20 rats led directly westward across the West 
Alley to the median barrier fence. This line of 
travel was directly toward the new source of food. 
Since the rats had been conditioned not to climb 
fences there was no attempt at climbing over and 
continuing toward this new goal. One might 
have suspected that the rats would have followed 
along the median barrier fence to Passage 4, 
since this response would have kept the olfactory 
stimulation at the highest level. However, with 
only one exception this did not occur. Instead 
all but one of the 20 rats turned around and 
retraced their steps to the outside of the Food 
Pen. From here they proceeded to the northwest 
outside corner of the Food Pen, and thence over 
the snow up Path 4 before heading through Area 
IV to the garbage. In this behavior of the rats 
there was apparently an integration of two systems 
or interactions of the organism with its environ- 
ment: A. There was the tendency of the rats to 
follow a gradient of odor to its point of origin. 
B. There was the tendency to adhere to customary 
routes of travel. Thus, the initial response of 
following the odor gradient was dominant until 
the rats met the barrier fence. At this point, 
without any wandering back and forth along the 
fence there was a suppression of this response 
to the extent that the rats returned to a known 
point of orientation, the northwest corner of the 
Food Pen, from which they could simultaneously 
respond to the gradient of olfactory stimulation 
while at the same time they were able to follow 
over a conditioned route of travel. However, 
it must be kept in mind that this previously traveled 
route had not been followed for at least 6 days. 
It will be helpful to refer to figure 71 in following 
this discussion. The dashed line in this figure 
shows the positions of the main trails existing on 
January 29. The arrow shows the direction of 
travel across the West Alley to the median barrier 
fence on the initial detection of the garbage. See 
figure 73 for additional detail. 
F. Utilization of Space by Rats. Observations upon 
marked rats indicate that over extended periods of 
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